I think it would be more accurate if it said Tiffany/free. I think it means they offer some free service and a Tiffany replica setting. If you scroll to the bottom of the page there is a heading that says Free and lists the prices of things like shipping and appraisals and Tiffany replica settings. Don't know if that is right, but it is my best guess!
Some diamond sellers include one of the inexpensive "tiffany" 4 or 6 prong settings free with their diamonds. We call them temp settings, usually, because most PSer's select another setting. They aren't what we call Tiffany repro settings because those are actual reproductions of the actual Tiffany solitaire setting.